Optical systems employing multiple mirrors to successively reflect beams of light are used to produce images of distant objects. Examples of such optical systems are telescopes and telescopic cameras. The mirrors in an optical system are typically a combination of curved and essentially flat mirrors.
A multiple-mirror optical system which works particularly well is a four-mirror system such as the four-mirror system 11 illustrated in FIG. 1. A four-mirror system is capable of producing an enhanced image because the four reflecting surfaces of such a system enable projection and formation of the image to be substantially controlled. A four-mirror system is thus more advantageous than a three- or two-mirror system. To reflect and propagate the beams 16, 17, 18 the four-mirror system 11 employs a primary mirror 12 which is a convex mirror, a secondary mirror 13 which is a concave mirror, a tertiary mirror 14 which is substantially flat, and a quaternary mirror 15 which is a concave mirror. After the beams 16, 17, 18 are reflected upon the mirrors 12, 13, 14, 15 the system produces an image of a desired object at a position of image production 19.
An objective in using a multiple-mirror optical system is to produce readily-perceivable images of distant objects from a large field of view. Often, large mirrors are used to attain a wide field of view. However, the greater the size of mirrors used, the more likely that the mirrors will obscure the beam pathway between mirrors. Thus, a problem in using multiple-mirror optical systems is that it is difficult to maintain proper alignment of the multiple mirrors so as to provide an unobscured pathway for reflected beams while also producing an image which is optimally focused. In the case of a four-mirror system 11 as illustrated, optimum imaging may be achieved but production and operation of the system is inhibited by the difficulty in maintaining an unobscured optical path when it is necessary to position four distinct mirrors. If an unobscured pathway is not maintained the ability of the optical system to produce an image will be adversely affected and the quality of the image produced will be adversely affected.
A similar problem in using multiple-mirror optical systems is that the quality of the image produced by the system is affected by the combined factors of the number, positioning and curvature of the mirrors which are necessary to enable the desired image to be produced. For example, although the four-mirror system has imaging advantages over other multiple-mirror systems, an image produced by the four-mirror system 11 is often highly distorted due to the number, positioning and curvature of the mirrors used.